Update cssnano

Author: mercs600

package.json

@@ -1,6 +1,6 @@
 {
   "name": "vue2-perfect-scrollbar",
-  "version": "1.5.55",
+  "version": "1.5.56",
   "description": "Vue.js wrapper for perfect scrollbar",
   "paths": {
     "source": "src",
@@ -73,7 +73,7 @@
     "webpack-dev-server": "^3.1.7"
   },
   "dependencies": {
-    "cssnano": "^4.1.3",
+    "cssnano": "^5.1.14",
     "perfect-scrollbar": "^1.5.5",
     "postcss-import": "^12.0.0"
   },